Home
Back
Weak Acid pH Approximation:
| From: | To: |
The weak acid pH approximation formula estimates the pH of a weak acid solution using the acid dissociation constant (Ka) and concentration (C). This approximation is valid when the acid is weak enough that the concentration of H⁺ ions is much smaller than the initial concentration of the acid.
The calculator uses the weak acid pH approximation formula:
Where:
Explanation: This formula assumes that the weak acid dissociation is small and that the concentration of H⁺ ions comes primarily from the dissociation of the weak acid.
Details: Accurate pH calculation is crucial for understanding acid-base chemistry, predicting chemical behavior, and applications in various fields including biochemistry, environmental science, and industrial processes.
Tips: Enter the acid dissociation constant (Ka) in mol/L and the concentration (C) in mol/L. Both values must be positive numbers greater than zero.
Q1: When is this approximation valid?
A: This approximation is valid when Ka << C, typically when Ka/C < 10⁻², meaning the acid is sufficiently weak.
Q2: What are typical Ka values for weak acids?
A: Weak acids typically have Ka values between 10⁻² and 10⁻¹⁰ mol/L. Examples include acetic acid (Ka ≈ 1.8×10⁻⁵) and hydrofluoric acid (Ka ≈ 6.8×10⁻⁴).
Q3: When should I use the exact quadratic formula instead?
A: Use the exact quadratic solution when Ka/C > 10⁻² or when higher precision is required, as the approximation becomes less accurate for stronger weak acids.
Q4: Does this account for water's autoionization?
A: No, this approximation neglects the contribution from water's autoionization, which becomes significant when [H⁺] < 10⁻⁶ M (pH > 6).
Q5: Can this be used for polyprotic acids?
A: No, this approximation is specifically for monoprotic weak acids. Polyprotic acids require more complex calculations that account for multiple dissociation steps.